Presently D dot and B dot sensor probes are used for high power microwave (HPM) tests and evaluations, wherein a D dot sensor is a dipole antenna based device capable of measuring an electric field, while a B dot sensor is a coil or loop based device used for measuring a magnetic field. Limitations and disadvantages of these conventional sensors include: (1) unacceptably large field perturbations, (2) narrow bandwidth, and (3) bulky physical size. Both the D dot and the B dot sensors perturb the very fields that they measure, based on the metallic composition of the sensors. The large field perturbations render these sensors unable to measure true waveform of the electric and magnetic fields. These conventional sensors have a very narrow frequency bandwidth (typically less than 1 GHz) and hence they are not suitable for wideband HPM test and evaluation. Also, the conventional electric field sensor (D dot sensor) is large in size, and thus unable to measure detailed field patterns in small areas.
The problems associated with conventional electric field sensors can be addressed with electro optic field sensors. Electro optic sensors are small in size and have large intrinsic bandwidths (dc to terahertz). Additionally, they contain no metallic parts and are therefore minimally perturbative to external electric fields. The principle of operation of an electro optic field sensors is based on the linear electro optic effect (also known as the Pockels effect), where an electric field modulates the birefringence of an electro optic material (electro optic crystal or electro optic polymer).
In an electro optic field sensor, a laser probe beam, analyzing optics and a photodetector are used to convert the modulation of birefringence (produced by the applied field and the electro optic effect in the crystal) into a modulated electrical output signal from which the applied field can be inferred. This process is accomplished as follows: In the absence of an applied field, the laser probe passes through the crystal and acquires a phase delay due to the natural birefringence of the crystal. The beam then exits the crystal and passes through an analyzer, consisting of a polarizer which may be preceded with one or more wave retardation plates. The amount of beam power transmitted through the analyzer depends on the phase delay (or equivalently, the polarization state) within the beam. When an external electric field is applied to the crystal, the modulation of birefringence results in a modulation in the phase (or equivalently, the polarization state) of the probe beam, thereby modulating the amount of beam power transmitted through the analyzer. Thus the phase-modulated laser beam is converted into an intensity-modulated optical output signal. By measuring the amplitude and phase of the output optical modulation signal using a photodetector and readout instruments (which convert the optical signal into an electrical signal), the amplitude and phase of the applied electric field can be determined.
However, a major problem with electro optic field sensors is that the optical modulation signal can undergo fluctuations and drifts in its amplitude and phase, even if the applied field has a steady state amplitude and phase. These fluctuations and drifts are due to temperature-induced and photo-induced changes in the natural birefringence of the electro optic material. Because of these fluctuations, precise field measurements become difficult or impossible. This instability problem is likely to be a major reason why little effort has been made to commercialize the use of electro optic field sensors in field measurement applications.
The fluctuations and drift problems in electro optic materials not only affect electro optic field sensors, but also electro optic modulators (which operate in a nearly identical manner as field sensors). In electro optic modulator applications, a known electric field is applied via electrodes to an electro optic material. A drive voltage is then applied across the electrodes, which induces a phase modulation in a probe beam passing through the electro optic material. Analyzing optics are then used to convert the phase modulation into an intensity modulated optical output signal. As in the case of electro optic sensors, drifts and fluctuations can occur in the amplitude and phase of the optical modulation signal, even if the drive voltage has a steady state amplitude and phase. As a result, measures must be taken to compensate for these fluctuations, such as varying the amplitude and phase of the drive voltage signal.
In electro optic field sensors and modulators, the fluctuations and drift in the optical output signal can be reduced by continuous adjustments of the rotation angle of the polarizer in the analyzing optics. Such a technique can improve the stability of the responsivity to a certain degree; however, continuous manual adjustments are cumbersome and can not always recover the signal loss. No device hitherto exists to eliminate these instabilities and keep the device stably operating at its peak responsivity.
Therefore, the need exists for an electric field sensor that is capable of nonperturbative measurements of the amplitude and phase of electric fields over a wide frequency bandwidth that can stably operate at its peak responsivity.
Further, the need exists for devices and systems directed to analyzing a phase modulated laser beam that has passed through an electro optic material in an electric field (produced externally or by a voltage difference across electrodes), and converting it into an intensity modulated optical signal such that the amplitude of the intensity modulation does not fluctuate or drift as the natural birefringence of the electro optic material varies.